The present invention generally relates to aqueous electrochemical cells and, more particularly, to an alkaline electrochemical cell having a gel-type electrode.
Conventional alkaline electrochemical cells generally include a cylindrical steel can having a positive electrode, commonly referred to as the cathode, which often comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, commonly referred to as the anode, which often comprises zinc powder as the active material. In conventional bobbin-type cell constructions, the anode is centrally located in a cavity within a tubularly shaped cathode. A cup-shaped separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is typically inserted into the anode active material, and a seal assembly, which commonly includes an annular polymeric seal, such as nylon, provides closure to the open end of the steel can to seal the active electrochemical materials in the can.
Some conventional alkaline cells commonly use a gelled anode which contains carboxymethyl cellulose (CMC), cross-linking type branched polyacrylic acid or a sodium salt thereof, natural gum, or the like as a gelling agent. Examples of conventional gelled anodes are disclosed in U.S. Pat. Nos. 5,587,254 and 4,963,447. The gelled anode is typically formed by uniformly dispersing irregularly shaped zinc powder in a gelled electrolyte containing a gelling agent, such as CMC, and an alkaline electrolyte solution, such as potassium hydroxide. The zinc powder is uniformly suspended in place by the gelling agent. The conventional gelled anode is generally preformed and injected in a gelled state into the cup-shaped separator with an injector. The gelled anode mix typically has a low viscosity and uniform concentration of zinc powder dispersed throughout the anode mix.
Zinc powder is commercially available in various particle shapes and sizes. Conventional zinc powders employed in alkaline batteries are generally characterized as irregularly shaped particles, ranging from lumps or distorted spheroids to elongated tuberous forms. These zinc particles typically have craggy or minor protrusions, and irregular surface characteristics. In conventional cells, the zinc powder is generally suspended in the gelling agent without respect to its orientation. The irregular shaped particles and non-uniform orientation of zinc particles may reduce ion permeation and water flow within the anode and may create void volume, thereby reducing available cell performance. The anode must include a minimal amount of zinc powder to provide sufficient particle-to-particle and particle-to-collector contact, yet not unnecessarily waste excessive zinc beyond the anode's reaction product capacity. Electrochemical cells have generally required a minimum of 28 percent volume of conventional zinc powder in the anode to provide sufficient electronic conductivity.
It is desirable to provide a zinc anode current carrying matrix that maintains good conductivity, high current discharge efficiency and allows for enhanced flow, high volume percent zinc concentrations, and improved packing efficiency.